1. Field of the Invention
The present invention relates to an ink jet printer head for a drop-on demand (DOD) type printer.
2. Description of the Prior Art
Among non-impact type printers, ink jet printers have recently become quite popular, due, in large part, to the fact that they operate on a relatively simple principle and are suitable for use in color printing. Of the non-impact type printers, continuous ink jet type printers were first developed, with DOD type printers being more recently developed. Such DOD type printers do not continuously jet ink, but rather jet ink only when it is necessary to form a dot. Currently, these DOD type printers are more popular than the continuous ink jet type printers.
A typical DOD type printer is a kizer type printer such as that disclosed in Japanese patent publication No. 12138/1978. However, such kizer DOD type printers are burdened by the fact that they are quite difficult to miniaturize.
Another typical DOD type printer is a thermal jet type such as that disclosed in Japanese patent publication No. 59914/1986. However, such thermal jet type printers are burdened by the fact that the ink used therein must be heated to a relatively high temperature, thus resulting in burning and sticking of the ink.
Accordingly, as disclosed in Japanese patent laid-open No. 252750/1988, a shear mode type DOD printer has been developed in order to overcome the above-noted problems confronting these typical DOD type printers. The construction and principles of operation of this shear mode type printer will now be described with reference to FIGS. 7-10 and 18.
As best shown in FIGS. 9(a) and 9(b), a plurality of elongated barriers 95ab, 95bc, and 95cd are bonded onto a base 105 by an adhesion layer 108 in such a manner as to form narrow slots 92a, 92b, and 92c which define ink chambers and flow paths. The ink for these narrow slots 92a, 92b and 92c is to be supplied from a common ink reservoir 187 defined at first ends of the slots 92a, 92b and 92c so as to be in communication, as best seen in FIGS. 10 and 18, with the narrow slots.
Second ends of the slots 95 are substantially closed by a nozzle plate 100 bonded to the ends of the barriers 95. The nozzle plate 100 has a plurality of small nozzle holes 93a-93f formed therein in communication with each of the slots 92a-92f, respectively.
A lid 106 is bonded to upper surfaces of the barriers 95 by a flexible elastic material 109 in such a manner that the barriers 95 are flexible in lateral directions relative to the lid 106 (see FIG. 9(b)).
The base 105 is to have electrical insulation characteristics by being formed, for example, of glass or ceramics. The lid 106 is also formed of glass or ceramics in order to provide it with electrical insulation characteristics. The barriers 95, however, are formed of piezoelectric material such as titanic acid zirconic lead (PZT).
Again referring to FIGS. 7, 9(a) and 9(b), electrodes 94a2-94f1 are mounted along the entirety of each of the side walls of the plurality of barriers 95ab-95ef. Each of the barriers 95ab-95ef is polarized in a like direction as shown by arrows 107 (or in a direction opposite thereto).
Accordingly, when a sufficiently large electric potential is induced across the electrodes 94a2 and 94b1, the barrier 95ab is forced to deflect in the manner shown in FIG. 9(b). As shown, because the elastic material 109 is more flexible than the adhesion layer 108, the deflection of the barrier 95ab mainly occurs at the upper portion thereof nearest the lid 106. In a like manner, when a sufficiently large electric potential is provided to the electrodes 94b1 and 94b2 (the electrodes 94b1 and 94b2 are normally of the same electric potential), the barrier 95bc is caused to deflect in the manner shown in FIG. 9(b). Such deflection of the barriers 95ab and 95bc causes a reduction in the cross-sectional area of the slot 92b (and thus in the volume thereof), such that ink contained in the slot 92b is forced outwardly through the nozzle hole 93b.
Thus, by selectively causing deflections of the various barriers in the above-noted manner, ink drops can be forced out (or jetted) from the selected nozzle holes 93a-93f.
With this type of arrangement, the slots 92a-92f may be formed narrowly so as to allow for miniaturization, and it is also unnecessary to utilize high temperatures as in the kizer type printer discussed above. Accordingly, the ink jet head disclosed in the Japanese patent application laid-open No. 252570/1988, the problems noted above in connection with DOD type printer heads of Japanese publication 12138/1978 and 59914/1986, have been obviated. However, this ink jet head disclosed in Japanese patent application laid-open No. 252750/1988 is still beset with various shortcomings.
More specifically, the reduction in cross section of each of the four slots 92b-92e is effected by deflection of the two barriers between which the particular slot is defined. However, this is not the case with respect to the two outermost slots 92a and 92f, the cross-sectional area of the slot 92a, for example, being effected by only the deflection of the barrier 95ab, and not by deflection of a second barrier. Therefore, if, when the cross-sectional area of the slot 92a is to be reduced in order to force an ink drop from the nozzle hole 93a, the barrier 95ab is caused to deflect toward the slot 92a by the same amount as each of the barriers 95ab and 95bc would be deflected toward the slot 92b in order to force an ink drop through the nozzle hole 93b, the force which will act upon the ink contained in the slot 92a will be less than that for the slot 92b. This can, in extreme cases, cause no ink to be discharged and, in other cases, can cause the dot created by the ink drop to be of a smaller or irregular size relative to dots produced from the nozzle holes 93b-93e. This results in poor printing quality due to the occurrence of missing ink dots and irregular ink dot sizes.
The reduction in the force acting on the ink in the slot 92a (or 92f) relative to that which acts on ink in the slots 92b-92e, can be somewhat obviated by applying different voltages to the outermost barriers 95ab and 95ef than is applied to the other barriers 95bc-95de. This variance in the voltage is applied as illustrated in FIG. 8, in which the vertical axis represents voltage and the horizontal axis represents time. The wave forms 81-86 in FIG. 8 represent different voltages applied to the barriers 95ab, 95bc and 95cd, respectively, at different times, and the lines 87, 88 and 89 represent zero voltage levels for the barriers 95ab, 95bc and 95cd, respectively.
As clearly illustrated in FIG. 8, the voltage applied to each barrier is opposite in polarity to that applied to its neighboring barrier, in order to cause the barriers to deflect toward or away from one another. The wave forms 81-86 also illustrate that application of voltage to the barriers is substantially instantaneous, whereas the removal of voltage from the barriers is relatively gradual. This is necessary so that the barriers are moved rapidly for the purpose of jetting ink, but moved more gradually in terminating the jetting of the ink. The wave forms 81-86 are thus shaped non-symmetrically in order to illustrate this manner of applying and removing the voltage from the barriers.
As further illustrated in FIG. 8, the magnitude of the voltage applied to the barrier 95ab to cause jetting of ink from the nozzle hole 93a is approximately double the magnitude of the voltage applied to each of the barriers 95ab and 95bc when it is desired to cause ink to be jetted from the nozzle hole 93b. This will increase the deflection of the barrier 95ab during jetting of ink from the nozzle hole 93a relative to the deflection of the two barriers 95ab and 95bc during jetting of ink from the nozzle hole 93b (in this regard, compare wave form 82 applied during jetting of ink from the nozzle hole 93a to the wave forms 81 and 83 illustrating the voltage applied during jetting of ink from the nozzle hole 93b).
With this application of a higher magnitude of voltage to the outermost barriers during jetting of ink from the outermost nozzle holes, the above-noted reduction in the ink jetting force from the nozzle holes 93a and 93f is at least partially obviated. However, this solution to the one problem results in additional problems as follows:
(1) Because the application of the higher voltage (as illustrated by wave form 82) causes a relatively greater deflection of the barrier 95ab, when ink is being jetted from the nozzle hole 93a, the cross-sectional area of the neighboring slot 92b is markedly increased, thus causing a substantial reduction in the pressure in the slot 92b. This reduction in pressure results in the formation of air bubbles in the ink contained in the slot 92b, thereby resulting in irregular jetting of ink from the nozzle hole 93b;
(2) Because the deflection of the barrier 95ab in forcing ink to be jetted from the nozzle 93a is relatively large, the return of the barrier 95ab to its normal rest position causes a relatively large volume reduction in the slot 92b, thereby often resulting in ink being improperly jetted from the nozzle hole 93b; and
(3) The non-symmetrical shape of the voltage wave forms 81 and 82, along with the large magnitude of the voltage of wave form 82, often results in the polarization of the barrier 95ab in the direction of the electrode 94b1 and away from the electrode 94a2. This polarization results in the reduction of deflecting force for the barrier 95ab.
In addition to the problems created by the fact that the outermost slots 92a and 92f are defined by only one barrier each, the shearing mode type ink jet printer head disclosed in Japanese patent application laid-open No. 252750/1988 is also beset with a problem which will now be described with particular reference to FIG. 18.
As shown in FIG. 18, the slots 92a-92f are substantially closed at ends thereof by the nozzle plate 100 having the nozzle holes 93 formed therein. During the manufacturing of the ink jet head, the placement and subsequent bonding of the nozzle plate 100 to the ends of the barriers 95 often results in the breakage of the end portions of the barriers 95, especially in view of the fact that the barriers 95 are formed of a piezoelectric material which is relatively brittle, and the fact that the barriers 95 are normally formed with a width of less than 100 .mu.m. Such breakage of the barriers 95 results in ink flowing between adjoining slots 92, such that deflection of a barrier for the purpose of jetting ink from one nozzle hole 93 may cause a rise in pressure in adjoining slots. In addition, such possible ink flow between the adjoining slots can result in the loss of pressure in a slot.